riscv/runtime-const: Disable linker relaxation for RUNTIME_MAGIC

Commit ee10b10281 ("riscv/runtime-const: Replace open-coded
placeholder with RUNTIME_MAGIC") switched the lui + addi[w] placeholder
from hand-encoded immediates to %hi()/%lo() of RUNTIME_MAGIC.

GNU Assembler folds %hi()/%lo() of an absolute expression at assembly
time. LLVM's integrated assembler since 21.1 defers it to the linker
instead, which range checks it: on riscv64 R_RISCV_HI20 must fit a
signed 20-bit field, and RUNTIME_MAGIC (0x89ABCDEF) is a positive 64-bit
value rather than a sign-extended 32-bit one, so it does not:

    ld.lld: error: relocation R_RISCV_HI20 out of range: 563901 is not in
    [-524288, 524287]

The sequence is patched at runtime and its instruction offsets are
recorded via ".long 1b - .", so the linker must not touch it in the
first place. Add ".option norelax", as is already done for
ALTERNATIVE() and static keys; the integrated assembler then resolves
%hi()/%lo() itself and emits no relocation, restoring the exact
encoding the open-coded placeholder produced.

Fixes: ee10b10281 ("riscv/runtime-const: Replace open-coded placeholder with RUNTIME_MAGIC")
Closes: https://github.com/ClangBuiltLinux/linux/issues/2179
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Fangrui Song <i@maskray.me>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260802174342.597092-1-i@maskray.me

arch/riscv/include/asm/runtime-const.h

@@ -23,6 +23,7 @@
 	typeof(sym) __ret;						\
 	asm_inline(".option push\n\t"					\
 		".option norvc\n\t"					\
+		".option norelax\n\t"					\
 		"1:\t"							\
 		"lui	%[__ret], %%hi(" RUNTIME_MAGIC ")\n\t"		\
 		"addi	%[__ret],%[__ret], %%lo(" RUNTIME_MAGIC ")\n\t"	\
@@ -47,6 +48,7 @@
 #define RISCV_RUNTIME_CONST_64_PREAMBLE				\
 	".option push\n\t"					\
 	".option norvc\n\t"					\
+	".option norelax\n\t"					\
 	"1:\t"							\
 	"lui	%[__ret], %%hi(" RUNTIME_MAGIC ")\n\t"		\
 	"lui	%[__tmp], %%hi(" RUNTIME_MAGIC ")\n\t"		\