Pass -C default-linker-libraries to rustc

By default, rustc passes -nodefaultlibs to the linker.  In some cases,
this can cause compiler builtins (e.g. __cpu_model, used by
__builtin_cpu_supports) not to be linked in:

= note: /nix/store/s2cjhni3s6shh0n35ay1hpj8d85k44qk-x86_64-unknown-linux-musl-binutils-2.43.1/bin/x86_64-unknown-linux-musl-ld: ./libbcachefs.a(tools-util.o): in function `resolve_crc32c':
        /build/source/c_src/tools-util.c:457:(.text+0x1023): undefined reference to `__cpu_model'
        collect2: error: ld returned 1 exit status

I saw this with static musl GCC builds, and others have reported it with
Clang.

Link: #300
Signed-off-by: Alyssa Ross <[email protected]>
Signed-off-by: Kent Overstreet <[email protected]>

Makefile

@@ -73,12 +73,13 @@ CFLAGS+=$(call cc-disable-warning, zero-length-array)
 CFLAGS+=$(call cc-disable-warning, shift-overflow)
 CFLAGS+=$(call cc-disable-warning, enum-conversion)
 CFLAGS+=$(call cc-disable-warning, gnu-variable-sized-type-not-at-end)
+export RUSTFLAGS=-C default-linker-libraries
 
 PKGCONFIG_LIBS="blkid uuid liburcu libsodium zlib liblz4 libzstd libudev libkeyutils"
 ifdef BCACHEFS_FUSE
 	PKGCONFIG_LIBS+="fuse3 >= 3.7"
 	CFLAGS+=-DBCACHEFS_FUSE
-	export RUSTFLAGS=--cfg fuse
+	RUSTFLAGS+=--cfg fuse
 endif
 
 PKGCONFIG_CFLAGS:=$(shell $(PKG_CONFIG) --cflags $(PKGCONFIG_LIBS))