Musl does support TLS in shared objects

[Hello71]

Description

f895e3bbf3076ed87fd87af3e4ea7de7db7b34a8 says "Musl libC doesn't support dynamically loaded binaries having TLS relocations", which is not correct. musl doesn't support dynamically loaded libraries with initial-exec TLS. dynamically loaded libraries need to use global-dynamic or local-dynamic TLS models, which can be selected in gcc by passing -fpic flag.

Example

https://godbolt.org/z/E4c3WWohq

static __thread void *cur_ex_ptr;
void **eh_get_exception_ptr(void) {
    return &cur_ex_ptr;
}

gcc -O2, bad:

eh_get_exception_ptr:
        mov     rax, QWORD PTR fs:0
        add     rax, OFFSET FLAT:cur_ex_ptr@tpoff
        ret
cur_ex_ptr:
        .zero   8

gcc -O2 -fpic, good:

eh_get_exception_ptr:
        sub     rsp, 8
        lea     rdi, cur_ex_ptr@tlsld[rip]
        call    __tls_get_addr@PLT
        add     rsp, 8
        add     rax, OFFSET FLAT:cur_ex_ptr@dtpoff
        ret
cur_ex_ptr:
        .zero   8

Note that __tls_get_addr may clobber callee-saved registers. -mtls-dialect=gnu2 can avoid that, but needs assembler+linker+libc support. glibc supports it since ~2008, so it's probably OK for MonoMod to use?