linux-user: set minimum uname for RISC-V

As support for RISC-V was only merged into the mainline kernel at 4.15
it is unlikely that glibc will be happy with a reported kernel version
of 3.8.0. Indeed when I testing binaries created by the current Debian
Sid compiler the tests failed with:

  FATAL: kernel too old

Bump the version to the minimum a RISC-V glibc would expect:

  https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/riscv/configure.ac

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180425100218.24785-1-alex.bennee@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

linux-user/riscv/target_syscall.h

@@ -45,7 +45,7 @@ struct target_pt_regs {
 #else
 #define UNAME_MACHINE "riscv64"
 #endif
-#define UNAME_MINIMUM_RELEASE "3.8.0"
+#define UNAME_MINIMUM_RELEASE "4.15.0"
 
 #define TARGET_MINSIGSTKSZ 2048
 #define TARGET_MLOCKALL_MCL_CURRENT 1