Failed to load program: last insn is not an exit or jmp

Caused by:
```
#!/usr/bin/env python3
import bcc
u = bcc.USDT(path='/lib64/libc.so.6')
u.enable_probe(probe='__v1setjmp', fn_name='foo')
b = bcc.BPF(text=r'''
#include

static int foo_1(struct pt_regs *ctx)
{
u64 arg;
bpf_usdt_readarg(1, ctx, &arg);
return 0;
}

int foo(struct pt_regs *ctx)
{
return foo_1(ctx);
}
''', usdt_contexts=[u], debug=65535)
```

Looking at generated C:
```
__attribute__((always_inline))
static __always_inline int _bpf_readarg_foo_1(struct pt_regs *ctx, void *dest, size_t len) {
if (len != sizeof(uint64_t)) return -1;
*((uint64_t *)dest) = ctx->gprs[2]; __asm__ __volatile__("": : :"memory");
return 0;
}
__attribute__((always_inline))
static __always_inline int _bpf_readarg_foo_2(struct pt_regs *ctx, void *dest, size_t len) {
if (len != sizeof(int32_t)) return -1;
*((int32_t *)dest) = ctx->gprs[3]; __asm__ __volatile__("": : :"memory");
return 0;
}
__attribute__((always_inline))
static __always_inline int _bpf_readarg_foo_3(struct pt_regs *ctx, void *dest, size_t len) {
if (len != sizeof(uint64_t)) return -1;
*((uint64_t *)dest) = ctx->gprs[14]; __asm__ __volatile__("": : :"memory");
return 0;
}

#include

__attribute__((always_inline))
static int foo_1(struct pt_regs *ctx)
{
u64 arg;
_bpf_readarg__1(ctx, &arg, sizeof(*(&arg)));
return 0;
}

__attribute__((section(".bpf.fn.foo")))
int foo(struct pt_regs *ctx)
{

return foo_1(ctx);
}
```

`foo_1` calls non-existent `_bpf_readarg__1`, which leads to the following BPF:

```
Disassembly of section .bpf.fn.foo:
foo:
; { // Line 43
0: bf 2a 00 00 00 00 00 00 r2 = r10
; int foo(struct pt_regs *ctx) // Line 42
1: 07 20 00 00 ff ff ff f8 r2 += -8
; _bpf_readarg__1(ctx, &arg, sizeof(*(&arg))); // Line 37
2: b7 30 00 00 00 00 00 08 r3 = 8
3: 85 01 00 00 ff ff ff ff call -1
; return foo_1(ctx); // Line 45
4: b7 00 00 00 00 00 00 00 r0 = 0
5: 95 00 00 00 00 00 00 00 exit
```

in which `call -1` apparently confuses the verifier.

Edit: this looks like #1255, but the error message has become extremely confusing.