gcc{10,11}: add no-sys-dir patch for RiscV

RiscV has an individual STARTFILE_PREFIX_SPEC macro which also need to
be patched.

pkgs/development/compilers/gcc/10/default.nix

@@ -64,6 +64,7 @@ let majorVersion = "10";
     patches =
          optional (targetPlatform != hostPlatform) ../libstdc++-target.patch
       ++ optional noSysDirs ../no-sys-dirs.patch
+      ++ optional (noSysDirs && hostPlatform.isRiscV) ../no-sys-dirs-riscv.patch
       /* ++ optional (hostPlatform != buildPlatform) (fetchpatch { # XXX: Refine when this should be applied
         url = "https://git.busybox.net/buildroot/plain/package/gcc/${version}/0900-remove-selftests.patch?id=11271540bfe6adafbc133caf6b5b902a816f5f02";
         sha256 = ""; # TODO: uncomment and check hash when available.

pkgs/development/compilers/gcc/11/default.nix

@@ -64,6 +64,7 @@ let majorVersion = "11";
     patches =
          optional (targetPlatform != hostPlatform) ../libstdc++-target.patch
       ++ optional noSysDirs ../no-sys-dirs.patch
+      ++ optional (noSysDirs && hostPlatform.isRiscV) ../no-sys-dirs-riscv.patch
       /* ++ optional (hostPlatform != buildPlatform) (fetchpatch { # XXX: Refine when this should be applied
         url = "https://git.busybox.net/buildroot/plain/package/gcc/${version}/0900-remove-selftests.patch?id=11271540bfe6adafbc133caf6b5b902a816f5f02";
         sha256 = ""; # TODO: uncomment and check hash when available.

pkgs/development/compilers/gcc/no-sys-dirs-riscv.patch

@@ -0,0 +1,12 @@
+--- a/gcc/config/riscv/linux.h
++++ b/gcc/config/riscv/linux.h
+@@ -69,8 +69,4 @@
+ 
+ #define TARGET_ASM_FILE_END file_end_indicate_exec_stack
+ 
+-#define STARTFILE_PREFIX_SPEC 			\
+-   "/lib" XLEN_SPEC "/" ABI_SPEC "/ "		\
+-   "/usr/lib" XLEN_SPEC "/" ABI_SPEC "/ "	\
+-   "/lib/ "					\
+-   "/usr/lib/ "
++#define STARTFILE_PREFIX_SPEC ""