add riscv64 architecture support

Author: Alex Suykov <alex.suykov@gmail.com>
Author: Aric Belsito <lluixhi@gmail.com>
Author: Drew DeVault <sir@cmpwn.com>
Author: Michael Clark <mjc@sifive.com>
Author: Michael Forney <mforney@mforney.org>
Author: Stefan O'Rear <sorear2@gmail.com>

This port has involved the work of many people over several years. I
have tried to ensure that everyone with substantial contributions has
been credited above; if any omissions are found they will be noted
later in an update to the authors/contributors list in the COPYRIGHT
file.

The version committed here comes from the riscv/riscv-musl repo's
commit 3fe7e2c75df78eef42dcdc352a55757729f451e2, with minor changes by
me for issues found during final review:

- a_ll/a_sc atomics are removed (according to the ISA spec, lr/sc
  are not safe to use in separate inline asm fragments)

- a_cas[_p] is fixed to be a memory barrier

- the call from the _start assembly into the C part of crt1/ldso is
  changed to allow for the possibility that the linker does not place
  them nearby each other.

- DTP_OFFSET is defined correctly so that local-dynamic TLS works

- reloc.h LDSO_ARCH logic is simplified and made explicit.

- unused, non-functional crti/n asm files are removed.

- an empty .sdata section is added to crt1 so that the
  __global_pointer reference is resolvable.

- indentation style errors in some asm files are fixed.

1 files changed, 14 insertions, 0 deletions

diff --git a/arch/riscv64/pthread_arch.h b/arch/riscv64/pthread_arch.h
new file mode 100644
index 00000000..1268b72d
--- /dev/null
+++ b/arch/riscv64/pthread_arch.h
@@ -0,0 +1,14 @@
+static inline struct pthread *__pthread_self()
+{
+	char *tp;
+	__asm__ __volatile__("mv %0, tp" : "=r"(tp));
+	return (void *)(tp - sizeof(struct pthread));
+}
+
+#define TLS_ABOVE_TP
+#define GAP_ABOVE_TP 0
+#define TP_ADJ(p) ((char *)p + sizeof(struct pthread))
+
+#define DTP_OFFSET 0x800
+
+#define MC_PC gregs[0]