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RISC-V: signal frame is misaligned in signal handlers
Description of problem:
`qemu-user` misaligns the signal frame (to 4 bytes rather than 16 bytes) on RISC-V 64, e.g causing pointer misalignment diagnostics to be triggered by UBSan.
Steps to reproduce:
1. Create a C file with the following contents:
```c
#include <signal.h>
#include <stdio.h>
void handler(int sig, siginfo_t *info, void *context) {
printf("signal occurred, info: %p, context: %p\n", info, context);
}
int main() {
struct sigaction act;
act.sa_flags = SA_SIGINFO;
act.sa_sigaction = handler;
sigaction(SIGINT, &act, NULL);
// Deliberately misalign the stack
asm volatile ("addi sp, sp, -4");
while(1);
// Unreachable
}
```
2. Compile with an appropriate RISC-V toolchain and run with `qemu-riscv64 ./a.out`.
3. Send a `SIGINT` (e.g by hitting Ctrl-C), and observe that the signal frame will be misaligned:
```
signal occurred, info: 0x400080025c, context: 0x40008002dc
```
Additional information:
This issue is alluded to in the source code, see https://gitlab.com/qemu-project/qemu/-/blob/master/linux-user/riscv/signal.c#L68-69. It should be sufficient to change that constant to 15.
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