guix: adjust RISC-V __has_include() patch to work with GCC 10

The actual macro is __has_include(), not __has_include__(), using the
later would result in build failures when using GCC 10. i.e:
```bash
../sysdeps/unix/sysv/linux/riscv/flush-icache.c:24:5: warning: "__has_include__" is not defined, evaluates to 0 [-Wundef]
   24 | #if __has_include__ (<asm/syscalls.h>)
```

Looks like at least someone else has run into the same thing, see:
http://lists.busybox.net/pipermail/buildroot/2020-July/590376.html.

See also:
https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005finclude.html
https://clang.llvm.org/docs/LanguageExtensions.html#has-include

contrib/guix/manifest.scm

@@ -553,7 +553,7 @@ inspecting signatures in Mach-O binaries.")
                (base32
                 "1b2n1gxv9f4fd5yy68qjbnarhf8mf4vmlxk10i3328c1w5pmp0ca"))
               (patches (search-our-patches "glibc-ldd-x86_64.patch"
-                                           "glibc-2.27-riscv64-Use-__has_include__-to-include-asm-syscalls.h.patch"))))))
+                                           "glibc-2.27-riscv64-Use-__has_include-to-include-asm-syscalls.h.patch"))))))
 
 (packages->manifest
  (append

contrib/guix/patches/glibc-2.27-riscv64-Use-__has_include-to-include-asm-syscalls.h.patch

@@ -1,3 +1,7 @@
+Note that this has been modified from the original commit, to use __has_include
+instead of __has_include__, as the later was causing build failures with GCC 10.
+See also: http://lists.busybox.net/pipermail/buildroot/2020-July/590376.html.
+
 https://sourceware.org/git/?p=glibc.git;a=commit;h=0b9c84906f653978fb8768c7ebd0ee14a47e662e
 
 From 562c52cc81a4e456a62e6455feb32732049e9070 Mon Sep 17 00:00:00 2001
@@ -59,7 +63,7 @@ index d612ef4c6c..0b2042620b 100644
  #include <atomic.h>
  #include <sys/cachectl.h>
 -#include <asm/syscalls.h>
-+#if __has_include__ (<asm/syscalls.h>)
++#if __has_include (<asm/syscalls.h>)
 +# include <asm/syscalls.h>
 +#else
 +# include <asm/unistd.h>