run:
"DEBUGGER=lldb __NV_PRIME_RENDER_OFFLOAD=1 __GLX_VENDOR_LIBRARY_NAME=nvidia DRI_PRIME=1 ./steam.sh"

got:

./steam.sh: line 117: VERSION_ID: unbound variable
./steam.sh: line 117: VERSION_ID: unbound variable
Running Steam on gentoo  64-bit
./steam.sh: line 117: VERSION_ID: unbound variable
STEAM_RUNTIME is enabled automatically
Updating Steam runtime environment...
--progress --auto-close --percentage=0 --no-cancel --width 400 --title=Steam setup --text=Updating Steam runtime environment...
Steam client's requirements are satisfied
WARNING: Using default/fallback debugger launch
lldb /Trinity/temp/ubuntu12_32/steam
(lldb) target create "/Trinity/temp/ubuntu12_32/steam"
Current executable set to '/Trinity/temp/ubuntu12_32/steam' (i386).
(lldb) start
error: 'start' is not a valid command.
(lldb) run
Process 133145 launched: '/Trinity/temp/ubuntu12_32/steam' (i386)
[2021-07-15 23:25:14] Startup - updater built Jul 12 2021 21:00:31
Installing breakpad exception handler for appid(steam)/version(1626127145)
Looks like steam didn't shutdown cleanly, scheduling immediate update check
[2021-07-15 23:25:14] Loading cached metrics from disk (/Trinity/temp/package/steam_client_metrics.bin)
[2021-07-15 23:25:14] Failed to load cached hosts file (File 'update_hosts_cached.vdf' not found), using defaults
[2021-07-15 23:25:14] Using the following download hosts for Public, Realm steamglobal
[2021-07-15 23:25:14] 1. http://media.steampowered.com, /client/, Realm 'steamglobal', weight was 1, source = 'baked in'
Installing breakpad exception handler for appid(steam)/version(1626127145)
[2021-07-15 23:25:14] Checking for update on startup
[2021-07-15 23:25:14] Checking for available updates...
[2021-07-15 23:25:14] Downloading manifest: http://media.steampowered.com/client/steam_client_ubuntu12
Installing breakpad exception handler for appid(steam)/version(1626127145)
[2021-07-15 23:25:15] Download skipped: /client/steam_client_ubuntu12 version 1626127145, installed version 1626127145, existing pending version 0
[2021-07-15 23:25:15] Nothing to do
[2021-07-15 23:25:15] Verifying installation...
[2021-07-15 23:25:15] Performing checksum verification of executable files
[2021-07-15 23:25:16] Verification complete
Loaded SDL version 2.0.15-6655637
Process 133145 stopped
* thread #1, name = 'steam', stop reason = signal SIGSEGV: invalid address (fault address: 0x0)
    frame #0: 0xf17e7330 libSDL2-2.0.so.0`___lldb_unnamed_symbol2620$$libSDL2-2.0.so.0 + 109
libSDL2-2.0.so.0`___lldb_unnamed_symbol2620$$libSDL2-2.0.so.0:
->  0xf17e7330 <+109>: movl   (%esi), %edx
    0xf17e7332 <+111>: testl  %edx, %edx
    0xf17e7334 <+113>: je     0xf17e7492                ; <+463>
    0xf17e733a <+119>: leal   -0x36ae2(%ebx), %eax
(lldb) 

replace the libSDL2 with 64bit lib I compiled got the same error

Now I know LTO broke the steam deps, but I don't know what package needs to rebuild with LTO disabel.

fix by reinstall system