linker_set: fix globl/weak symbol redefinitions to work on clang 12
In clang 12.0.0.rc2, going from weak to global is now a hard error:
/usr/src/stand/libsa/amd64/_setjmp.S:67:25: error: _longjmp changed binding to STB_GLOBAL .text; .p2align 4,0x90; .globl _longjmp; .type _longjmp,@function; _longjmp:; .cfi_startproc
And the other way is a warning, but we have -Werror:
error: __start_set_Xcommand_set changed binding to STB_WEAK [-Werror,-Winline-asm] error: __stop_set_Xcommand_set changed binding to STB_WEAK [-Werror,-Winline-asm]
ref: https://reviews.llvm.org/D90108
Reviewed By: arichardson
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29159