Mention the required GLIBC version in help and release note

Author: kawashima-fj

clang/include/clang/Driver/Options.td

@@ -3487,7 +3487,8 @@ def fno_experimental_isel : Flag<["-"], "fno-experimental-isel">, Group<f_clang_
   Alias<fno_global_isel>;
 def fveclib : Joined<["-"], "fveclib=">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
-    HelpText<"Use the given vector functions library">,
+    HelpText<"Use the given vector functions library.\n"
+             "  Note: -fveclib=libmvec on AArch64 requires GLIBC 2.40 or newer.">,
     HelpTextForVariants<[ClangOption, CC1Option],
       "Use the given vector functions library.\n"
       "  Note: -fveclib={ArmPL,SLEEF,libmvec} implies -fno-math-errno.\n"

flang/docs/ReleaseNotes.md

@@ -34,7 +34,8 @@ page](https://llvm.org/releases/).
 
 * -floop-interchange is now recognized by flang.
 * -floop-interchange is enabled by default at -O2 and above.
-* -fveclib=libmvec is supported for AArch64 (same as Flang/x86 and Clang/AArch64)
+* -fveclib=libmvec is supported for AArch64 (same as Flang/x86 and
+  Clang/AArch64) (requires GLIBC 2.40 or newer)
 
 ## Windows Support