um: avoid struct sigcontext redefinition with musl

mcontext.c includes both <sys/ucontext.h> and <asm/sigcontext.h>.
With musl libc, this causes a struct sigcontext redefinition error:

  <sys/ucontext.h> pulls in musl's <bits/signal.h>, which defines
  struct sigcontext directly.  The kernel's <asm/sigcontext.h> then
  provides a second, conflicting definition of the same struct.

With glibc this does not conflict because glibc's signal headers
source their struct sigcontext from the kernel's own UAPI headers,
so the include guard in <asm/sigcontext.h> makes the second
inclusion a no-op.

mcontext.c does not actually use struct sigcontext by name -- it
only needs the FP-state types (_fpstate, _xstate, etc.) that are
defined in <asm/sigcontext.h> independently of the sigcontext
struct.

Temporarily rename sigcontext to __kernel_sigcontext during the
inclusion of <asm/sigcontext.h> so that the kernel's definition
does not collide with musl's.  The #undef restores normal name
resolution immediately afterward.

No functional change with glibc; fixes the build with musl.

Signed-off-by: Marcel W. Wysocki <maci.stgn@gmail.com>
Link: https://patch.msgid.link/20260215142803.1455757-2-maci.stgn@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

arch/x86/um/os-Linux/mcontext.c

@@ -4,7 +4,13 @@
 #include <linux/string.h>
 #include <sys/ucontext.h>
 #include <asm/ptrace.h>
+/*
+ * musl defines struct sigcontext in <bits/signal.h>.  Rename the kernel's
+ * copy to avoid redefinition while keeping the FP-state types available.
+ */
+#define sigcontext __kernel_sigcontext
 #include <asm/sigcontext.h>
+#undef sigcontext
 #include <sysdep/ptrace.h>
 #include <sysdep/mcontext.h>
 #include <arch.h>