[riscv-port] RFR: 8278890: riscv: Missing features string in VM_Version

[Yadong Wang]

The features_string of VM_Version is supposed to be "sifive,u74-mc,rv64imafdc" for the unmatched board, which is made up of "uarch" and "isa" in /proc/cpuinfo.
In contrast, it's "family 6 model 79 stepping 1 microcode 0xb000036, cx8, cmov, fxsr, ht, mmx, 3dnowpref, sse, sse2, sse3, ssse3, sse4.1, sse4.2, popcnt, lzcnt, tsc, tscinvbit, avx, avx2, aes, erms, clmul, bmi1, bmi2, rtm, adx, fma, vzeroupper, clflush" in a x86 platform, and "0x41:0x0:0xd08:2, fp, simd, evtstrm, aes, pmull, sha1, sha256, crc" in aarch64.
After this PR, the following output is displayed when you run this test case `test/jdk/jdk/jfr/event/os/TestCPUInformation.java` on some common riscv platforms:
1. unmatched:

Event: jdk.CPUInformation {
  startTime = 15:54:00.844 (2021-12-20)
  cpu = "RISCV64"
  description = "RISCV64 sifive,u74-mc,rv64imafdc"
  sockets = 4
  cores = 4
  hwThreads = 4
}

2. D1:

Event: jdk.CPUInformation {
  startTime = 17:04:39.520 (2021-12-20)
  cpu = "RISCV64"
  description = "RISCV64 rv64imafdcv"
  sockets = 1
  cores = 1
  hwThreads = 1
}

-------------

Commit messages:
 - 8278890: riscv: Missing features string in VM_Version

Changes: https://git.openjdk.java.net/riscv-port/pull/35/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=35&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278890
  Stats: 49 lines in 3 files changed: 29 ins; 6 del; 14 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/35.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/35/head:pull/35

PR: https://git.openjdk.java.net/riscv-port/pull/35