fix support for initialized TLS in static PIE binaries

the static-linked version of __init_tls needs to locate the TLS
initialization image via the ELF program headers, which requires
determining the base address at which the program was loaded. the
existing code attempted to do this by comparing the actual address of
the program headers (obtained via auxv) with the virtual address for
the PT_PHDR record in the program headers. however, the linker seems
to produce a PT_PHDR record only when a program interpreter (dynamic
linker) is used. thus the computation failed and used the default base
address of 0, leading to a crash when trying to access the TLS image
at the wrong address.

the dynamic linker entry point and static-PIE rcrt1.o startup code
compute the base address instead by taking the difference between the
run-time address of _DYNAMIC and the virtual address in the PT_DYNAMIC
record. this patch copies the approach they use, but with a weak
symbolic reference to _DYNAMIC instead of obtaining the address from
the crt_arch.h asm. this works because relocations have already been
performed at the time __init_tls is called.

src/env/__init_tls.c

@@ -71,6 +71,9 @@ typedef Elf32_Phdr Phdr;
 typedef Elf64_Phdr Phdr;
 #endif
 
+__attribute__((__weak__, __visibility__("hidden")))
+extern const size_t _DYNAMIC[];
+
 static void static_init_tls(size_t *aux)
 {
 	unsigned char *p;
@@ -83,6 +86,8 @@ static void static_init_tls(size_t *aux)
 		phdr = (void *)p;
 		if (phdr->p_type == PT_PHDR)
 			base = aux[AT_PHDR] - phdr->p_vaddr;
+		if (phdr->p_type == PT_DYNAMIC && _DYNAMIC)
+			base = (size_t)_DYNAMIC - phdr->p_vaddr;
 		if (phdr->p_type == PT_TLS)
 			tls_phdr = phdr;
 	}