add rcrt1 start file for fully static-linked PIE

static-linked PIE files need startup code to relocate themselves, much
like the dynamic linker does. rcrt1.c reuses the code in dlstart.c,
stage 1 of the dynamic linker, which in turn reuses crt_arch.h, to
achieve static PIE with no new code. only relative relocations are
supported.

existing toolchains that don't yet support static PIE directly can be
repurposed by passing "-shared -Wl,-Bstatic -Wl,-Bsymbolic" instead of
"-static -pie" and substituting rcrt1.o in place of crt1.o.

all libraries being linked must be built as PIC/PIE; TEXTRELs are not
supported at this time.

Makefile

@@ -44,7 +44,7 @@ ALL_INCLUDES = $(sort $(wildcard include/*.h include/*/*.h) $(GENH) $(ARCH_INCLU
 
 EMPTY_LIB_NAMES = m rt pthread crypt util xnet resolv dl
 EMPTY_LIBS = $(EMPTY_LIB_NAMES:%=lib/lib%.a)
-CRT_LIBS = lib/crt1.o lib/Scrt1.o lib/crti.o lib/crtn.o
+CRT_LIBS = lib/crt1.o lib/Scrt1.o lib/rcrt1.o lib/crti.o lib/crtn.o
 STATIC_LIBS = lib/libc.a
 SHARED_LIBS = lib/libc.so
 TOOL_LIBS = lib/musl-gcc.specs
@@ -89,7 +89,7 @@ src/ldso/dlstart.lo src/ldso/dynlink.lo: src/internal/dynlink.h arch/$(ARCH)/rel
 
 crt/crt1.o crt/Scrt1.o src/ldso/dlstart.lo: $(wildcard arch/$(ARCH)/crt_arch.h)
 
-crt/Scrt1.o: CFLAGS += -fPIC
+crt/Scrt1.o crt/rcrt1.o: CFLAGS += -fPIC
 
 OPTIMIZE_SRCS = $(wildcard $(OPTIMIZE_GLOBS:%=src/%))
 $(OPTIMIZE_SRCS:%.c=%.o) $(OPTIMIZE_SRCS:%.c=%.lo): CFLAGS += -O3

crt/rcrt1.c

+#define SHARED
+#define START "_start"
+#define _dlstart_c _start_c
+#include "../src/ldso/dlstart.c"
+
+int main();
+void _init() __attribute__((weak));
+void _fini() __attribute__((weak));
+_Noreturn int __libc_start_main(int (*)(), int, char **,
+	void (*)(), void(*)(), void(*)());
+
+_Noreturn void __dls2(unsigned char *base, size_t *sp)
+{
+	__libc_start_main(main, *sp, (void *)(sp+1), _init, _fini, 0);
+}