main in a library regression

recent wasi-libc is broken if you put main in a library.

test case: https://github.com/yamt/garbage/tree/d422b1073125b3fc8f843ce4b12280ab3982fb84/c/main_in_lib

spacetanuki% ./build-native.sh
spacetanuki% ./a.out
this is foo
spacetanuki% WASI_SDK=/opt/wasi-sdk-20.0 ./build-wasi.sh
spacetanuki% toywasm --wasi a.out
this is foo
spacetanuki% WASI_SDK=/opt/wasi-sdk-21.0 ./build-wasi.sh
spacetanuki% toywasm --wasi a.out                       
Error: [trap] unreachable executed (4): unreachable at 0001a5
frame[  2] funcpc 0001a5 (<unknown>:undefined_weak:main) callerpc 0022de
  param [0] = 00000001
  param [1] = 00010b30
frame[  1] funcpc 00226c (<unknown>:__main_void) callerpc 0001df
  local [0] = 00010b00
  local [1] = 00010b30
  local [2] = 00010b20
frame[  0] funcpc 0001ab (<unknown>:_start)
  local [0] = 00000000
2024-03-16 14:47:15 (1710568035.864123000): [117042600] instance_execute_func failed with -1
2024-03-16 14:47:15 (1710568035.864618000): [117042600] invoke failed with -1
spacetanuki% 

i guess this is a regression caused by https://github.com/WebAssembly/wasi-libc/pull/429

Start with the c/main_in_lib test case and run build-native.sh and build-wasi.sh using WASI SDK 20.0 and 21.0 to confirm the regression. Inspect the behavior associated with wasi-libc pull request 429 and the undefined_weak:main entry point; done means the WASI-built program prints "this is foo" without trapping.