systemd-boot: Pin to use gcc toolchain on arm

clang does not support -mgeneral-regs-only option for arm

Cc: Ross Burton <Ross.Burton@arm.com>
(From OE-Core rev: 553f769c961ca567a68ac98a5262d84a95eca596)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-core/systemd/systemd-boot_257.5.bb

@@ -54,6 +54,9 @@ TUNE_CCARGS:remove = "-mfpmath=sse"
 
 CFLAGS:append:libc-musl = " -D__DEFINED_wchar_t"
 
+# arm-poky-linux-musleabi-clang: error: unsupported option '-mgeneral-regs-only' for target 'arm-poky-linux-musleabi'
+TOOLCHAIN:arm = "gcc"
+
 COMPATIBLE_HOST = "(aarch64.*|arm.*|x86_64.*|i.86.*|riscv.*)-linux"
 COMPATIBLE_HOST:x86-x32 = "null"