EOF is not detected, when semihosting is reading from stdin
Description of problem:
QEMU hangs.
Steps to reproduce:
1. Run the program with stdin from a pipe.
Additional information:
The code is compiled from this source:
```
#include <stdio.h>

int main(int argc, char** argv) {
    int i = -1;
    int result = scanf("%d", &i);
    printf("result = %d, i = %d\n", result, i);
    return 0;
}
```
compiled with GCC and picolibc:
```
arm-none-eabi-gcc --specs=picolibc.specs -march=armv7-m ~/sources/picolibc/git/test-stdin.c -o test-stdin -lc -lsemihost --crt0=hosted -O0 -g
```
[test-stdin](/uploads/dbd2650c8e0aaca353fd7630ac9c8440/test-stdin)
The execution hangs at semihosting SYS_READC(0x7) call:
```
	movs r0, #7
(...)
	bkpt #0xab
```