linux-user/alpha: populate AT_HWCAP from env->amask

Alpha has never set AT_HWCAP in linux-user emulation, so getauxval(AT_HWCAP)
always returned 0 regardless of the emulated CPU model.

The Linux kernel computes ELF_HWCAP as ~amask(-1), i.e. the set of ISA
extension bits that the amask instruction reports as supported (cleared in
its output).  env->amask stores exactly those bits with the same layout
(BWX=0x1, FIX=0x2, CIX=0x4, MVI=0x100, TRAP=0x200, PREFETCH=0x1000), so
returning it directly from get_elf_hwcap matches the kernel convention.

Add HAVE_ELF_HWCAP to target_elf.h and implement get_elf_hwcap() in
elfload.c to expose the emulated CPU's capability mask to user-space
programs via the auxiliary vector.

Without this fix, programs using getauxval(AT_HWCAP) to detect BWX/FIX/CIX
(such as glibc's memcpy or JIT compilers targeting Alpha) incorrectly
concluded that no extensions were available even when emulating ev56+.

Signed-off-by: Matt Turner <mattst88@gmail.com>
Cc: qemu-stable@nongnu.org
Reviewed-by: Helge Deller <deller@gmx.de>
Signed-off-by: Helge Deller <deller@gmx.de>

linux-user/alpha/elfload.c

@@ -6,6 +6,17 @@
 #include "target_elf.h"
 
 
+abi_ulong get_elf_hwcap(CPUState *cs)
+{
+    /*
+     * The Linux kernel computes ELF_HWCAP as ~amask(-1), which clears a bit
+     * for each supported ISA extension.  env->amask stores exactly those bits
+     * set for the extensions supported by the emulated CPU model, matching
+     * the kernel's convention: bit set in AT_HWCAP ↔ extension present.
+     */
+    return cpu_env(cs)->amask;
+}
+
 void elf_core_copy_regs(target_elf_gregset_t *r, const CPUAlphaState *env)
 {
     int i;

linux-user/alpha/target_elf.h

@@ -12,6 +12,7 @@
 #define ELF_MACHINE             EM_ALPHA
 
 #define HAVE_ELF_CORE_DUMP      1
+#define HAVE_ELF_HWCAP          1
 
 /*
  * Matches the kernel's elf_gregset_t (ELF_NGREG = 33):